METAMORPHISM OF DABIE MOUNTAINS AND ITS RELATIONSHIP WITH THE COLLISIONAL OROGENY

The Dabie orogenic belt can be divided into 5 meta-tectonic units from south to north,i.e.,the northern Yangtze blueschist belt,Susong metamorphic complex,southern Dabie collisional complex,northern Dabie metamorphic complex and northern Huaiyang metamorphic belt. The rocks of these different units have undergone three types of metamorphism:(1) The ultrahigh pressure type is repressented by the coesite (and/or diamond)-bearing eclogites and only found in the southern Dabie collisional complex,the matamorphic PT path of which implies an oceanic B-type subduction.(2) The high pressure type occurs in the blueschist belt,Susong complex and metasediments and some gneisses in the southern Dabie complex and is probably related with the continental A-tpe subduction of Yangtze Plate.The increase of metamorphic grades from south to north suggests the increase of depth in this direction of subduction. (3) The medium-low pressure metamorphism widespread in the Dabie Mountains may be related to the thermal relaxation in thickened crustal regions caused by large-scale collision,with the metamorphic grade being as high as amphibolite facies (locally to granulite facies) in the northern Dabie complex,and declining southwards and northwards, to epidote amphibolite facies and greenschist facies. The boundaries between different facies are consistent with the strike-slip faults and decollement faults of a late stage. This late stage of meium-low metamorphism has modified to different extents the high and ultrahigh pressure metamorphic rocks of early stages. The southern Dabie collisional complex comprises rocks at least of these three metamorphic types having different metamorphic histories or PT paths. These three type of metamorphism together with the spatial distribution of metamorphic facies belts in Dabie Moutains give the orogenic evolution through oceanic B-type subduction-continental A-type subduction-thrusting arising from continent-continent collision to decollement due to uplifiting.